Previously, there have been challenges in precisely controlling certain critical dimensions in printed electrically functional features (and especially high resolution dielectric, conductor and semiconductor features). Conventional printing process conditions have resulted in poor control of length, height (thickness), cross-sectional profile and width of printed lines and islands. Thus, an ideal profile for printed features in a high performance device such as a thin film transistor (TFT) has been difficult to achieve.
Conventional printing processes may rely on an absorbing substrate (e.g., paper or cloth) to fix a position and a size of a deposited material (e.g., an ink). However, substrates typically used in manufacturing electronic devices are generally non-absorbing. The ink, as printed on a non-absorbing substrate, will behave as a liquid and will tend to move and/or spread until (or unless) the solvent is evaporated. Typically, the evaporation rate of the deposited ink is greatest near its edge, and liquid from the bulk of the drop tends to flow to the edge as evaporation occurs, resulting in deposition of solute particles near the edge. This phenomenon is sometimes referred to as “coffee ring” formation. The coffee ring profile is undesirable for semiconductor, conductor and/or dielectric structures in microelectronic applications, and there is a need for printing processes that form semiconductor, conductor and dielectric features having a more uniformly distributed shape (e.g. a dome-shaped profile), which is critical to obtaining high performance devices.
Organic electronic printing has been done on solid substrates using surface energy patterning and/or printing into predefined structures (formed by photolithography or another radiation-based patterning process) such as wells. In an all-additive, all-printed process, these methods preferably are not used. In order to fix the anisotropic shape (typically a line) of the active (e.g., Si) components, the line must be fixed or “pinned.” Without a mechanism for pinning the liquid as the solvent evaporates, the liquid will generally retreat until it forms one or more spherical drops on the surface, rather than a line or pattern.